1. Field of the Invention
The present invention relates in general to a control apparatus for a vehicular drive system, and more particularly to a control apparatus for a hybrid vehicle drive system including an electric motor, which control apparatus is configured to prevent excessive speeds of rotary elements such as the electric motor.
2. Discussion of Prior Art
There is known a hybrid type vehicle including (a) an electrically controlled differential portion including a differential mechanism, a first electric motor operatively connected to a rotary element of the differential mechanism, a drive power source, an input shaft connected to the drive power source, and an output shaft, a differential state between rotating speeds of the input and output shafts being controlled by controlling an operating state of the first electric motor, and (b) a second electric motor connected to a power transmitting path between the output shaft and drive wheels of the hybrid type vehicle. JP-2005-264762A discloses an example of a control apparatus for such a hybrid type vehicle. This publication discloses a technique for operating the first and second electric motors in the same direction upon starting of the drive power source in the form of an engine, to rapidly raise the operating speed of the engine to a level at which the engine can be started by ignition.
According to the control apparatus for the vehicular drive system as disclosed in the above-identified publication, the rotating speed of the output shaft of the electrically controlled differential portion may become excessively high exceeding a permissible maximum value in the absence of a load acting on the output shaft, when the power transmitting path is switched to a power cut-off state in the form of a neutral state during high-speed running of the vehicle. When the rotating speed of the output shaft of the electrically controlled differential portion becomes excessively high, not only an input rotary member of the power transmitting path connected to the output shaft, but also the second electric motor connected to the input rotary member are rotated at excessively high speeds, giving rise to a risk of deterioration of the durability of the input rotary member and second electric motor.